The market for light emitting diodes (LEDs) has a high potential in the global general lighting market. Light management systems and color control of LED light, which can affect the mood of the end users, enabling a purpose-friendly ambience, will shape the market into a new sphere.
There is currently a strong demand for color (RGB) and tunable white LED lamps with reduced costs and smaller form factor. Many companies are interested in producing and selling innovative and cost-effective solutions for driving multiple LED strings in large panel displays or general lighting applications. However, the existing topologies for driving multiple LED strings are very inefficient. They typically require multiple DC-DC converters and complex circuitry which increase the total component count and overall bill of material costs. The cost and complexity increases with the total number of LED strings.
Existing topologies for driving multiple LED strings are very inefficient. They typically require multiple DC-DC converters and complex circuitry, which increase the total component count and overall material costs. Further, the cost and complexity increases with the total number of LED strings.
Conventional AC-DC LED drivers consist of an AC-DC converter followed by multiple constant-current DC-DC converters, one for each LED string. FIG. 1 shows the simplified system architecture of this traditional AC-DC LED driver.
As depicted in FIG. 1, there is an AC-DC converter 10 driven by the AC mains 11. The AC-DC converter in turn drives DC-DC converters 12, one for each LED string 14. The number of DC-DC converters scales, i.e., increases, with the number of LED strings. This will inevitably lead to higher costs and a larger form factor as the total number of LED strings increases.
Recent research has proposed a DC-DC LED driver which can reduce the number of DC-DC converters (i.e. the number of inductors) by combining them using a single-inductor multiple-output (SIMO) DC-DC converter 16 and providing power using a time-multiplexing control. In general, they can be represented by the system architecture shown in FIG. 2. Compared with the conventional driver topology, the SIMO topology offers a simple, scalable and low-cost solution since it uses only a single inductor L to drive multiple independent LED strings.
The LED driver shown in FIG. 2 is powered by a DC voltage source 15. All the LED strings are driven by the common SIMO DC-DC switching converter 16. The energy from the inductor in the power stage of the switching regulator is distributed across the LED strings in a time-multiplexed manner. The benefit of this SIMO topology is that only one single inductor L is needed to drive multiple outputs. A pulse width modulation controller is used to determine the ON and OFF timing of the power and output switches primarily using current-sense voltage as the feedback signal. This kind of battery-powered DC-DC SIMO is mostly useful for low-power portable lighting applications.
An AC-DC converter 10 can be added in front of the aforementioned DC-DC SIMO 16 to form a two-stage AC-DC SIMO driver. This particular type of AC-DC converter generates an unregulated DC voltage across a capacitor 17 at its output, which output becomes the input voltage for the subsequent DC-DC converter 16. FIG. 3 shows the system architecture of this existing two-stage AC-DC SIMO LED driver. Notice that this AC-DC SIMO driver does not incorporate power factor correction (PFC).
An AC-DC multi-channel SIMO LED driver with PFC was reported in the literature in 2014. Basically, this is a two-stage driver topology where the first stage is a boost converter 20 with power-factor correction (PFC) 22 and the second stage is a buck converter 16′, which distributes identical DC current across multiple LED channels using a single inductor L. FIG. 4 shows the system architecture for this kind of two-stage SIMO LED driver.
It is evident that the control logic for this two-stage system is quite complicated as it involves a front-end and back-end controller. A major drawback for this particular AC-DC SIMO LED driver is that it does not support unequal currents across LED strings, i.e. so-called unbalanced loads. In addition, the fact that it operates in a continuous conduction mode (CCM) and employs only a single energizing phase per switching cycle means that it suffers from serious cross-regulation among the LED strings. A load transient in one LED channel will inevitably affect the DC operating point in other unchanged LED channels.